The invention relates to a process for the non-contacting measurement while moving of the thickness and temperature of thin metal sheets by means of Foucault or eddy currents.
The man skilled in the art is aware of the advantages in being able to measure the thickness of metal sheets issuing from a rolling mill, while the metal sheets are moving along, so as to be able to regulate the rolling parameters within limits which make it possible either to maintain a constant thickness of metal sheet or to vary the thickness of the metal sheet as desired, according to user needs.
That problem arises in particular when rolling thin sheets in particular of aluminium in a range of thicknesses of between 8 and 2000 .mu.m.
Due to the increase in rolling speeds, in order to make such measurements, recourse is preferably had to apparatuses in which the measuring element is not in contact with the sheet in order not to leave any mark nor to cause problems in the measuring operation by virtue of the measuring element suffering from progressive wear.
That is the case for example with apparatuses using eddy currents or radiation in which the measuring element operates at a certain distance from the sheet.
Among such arrangements, that which uses eddy currents has the advantage of being simple, low in cost and high in reliability.
The principle thereof is as follows:
A primary winding is supplied with an alternating signal by an oscillator; that winding therefore generates an alternating magnetic field which induces a current in a winding which is referred to as a secondary winding, using the same principle as a transformer, and develops an electrical voltage therein.
When a metal sheet is placed between the windings, the magnetic field generated by the first winding induces a current in the sheet. That current in turn generates a field which opposes that which gave rise thereto. The consequence thereof is to cause the voltage in the second winding to fall. If a thicker sheet is set in position between the windings, the induced currents are higher and the voltage drops even more in the secondary winding. It is thus possible to measure the thickness of the aluminium sheet in the course of rolling in a contact-less manner by measuring the voltage developed in the secondary winding.
However that type of measuring operation suffers from a major disadvantage: the induced current is proportional to the product .sigma..times.e (a relationship established by Forster) in which .sigma. is the conductivity of the sheet and e is its thickness. Now, .sigma. depends not only on the composition of the metal and its structure but also its temperature. If in the courco of rolling a strip, it is possible to ensure that the product is homogenous, that is not true in regard to temperature.
In fact, it is known that, in the course of an operation of rolling aluminium sheets, the temperature parameter may vary by several tens of degrees. That means that such a process, when applied to measurement in respect of thickness at the discharge of a rolling mill will give rise to erroneous measurements by virtue of the interference of the effects of temperature.